Superluminous supernovae spotted in the early Universe

Explosions triggered by light being converted into matter/antimatter particles.

A simulation showing the complex environment that would host a pair-instability supernova in the early Universe.

Adrian Malec and Marie Martig (Swinburne University).

For a variety of reasons, many think the first stars to form in the Universe were monsters, hundreds of times the mass of the Sun. With that much mass, the stars were destined to have short lives that ended violently in a supernova, seeding the first galaxies with the elements that would form future stars and planets. There have been a few reports of gamma-ray bursts that are distant enough that they probably signal the end of the first generation of stars, but we have not observed any of these supernovae directly.

Now, researchers have used a survey that ran for several years to identify distant superluminous supernovae—explosions that are so bright they are thought to occur when the most massive stars convert light into matter/antimatter particle pairs at their core. Although these don't appear to have come from the explosion of the Universe's first stars, the more distant of the events dates from only 1.5 billion years after the Big Bang. Based on this success, they suggest their approach can be used to spot the demise of the Universe's first stars.

Why were these stars so big? A star is a balancing act between gravity and energy. Too much energy, and the gas cloud that could form a star would dissipate. Too much gravity, and it will quickly collapse into a black hole. As the gas cloud collapses into a star, this collapse liberates energy, which threatens to push the cloud apart if it can't be radiated away. And, as it turns out, pure hydrogen (which is most of the early Universe) is lousy at dissipating this energy. So it's really hard to form a star out of nothing but hydrogen (the complex mix of elements present after the first stars exploded makes the process easier).

To get around this inefficiency, the first stars formed from very dense gas clouds, where there was enough gravity to overcome their internal heat. Models indicate the results were enormous, 200 to 300 times the mass of the Sun, and they burned through their hydrogen quickly.

Theoretical considerations suggested stars with this mass would suffer a rather unusual fate. At their cores, the density of gamma-ray light would reach levels that were so high, some of it would spontaneously convert into pairs of particles and antiparticles (remember, e = mc2, so the energy of the gamma-rays can be converted to matter). Having antimatter at the core of the star was bad enough, but the sudden loss of radiation would essentially pull the legs out from under the core of the star. Without the steady outward pressure from the energy, the entire core of the star would suddenly collapse.

With stars this size, that's a rather extreme event. About 60 solar masses worth of carbon and oxygen fuse into heavier elements almost instantaneously. The resulting thermonuclear explosion tears the star to pieces, leaving nothing behind but a rapidly expanding gas cloud.

For years, theorists were waiting for astronomers to catch up with them and observe one of these supernovae, termed "pair instability." In recent years, they seem to have done so, spotting a couple examples of supernovae that reach extreme brightness, 10 times more luminous than the strongest type Ia explosions. They also have a distinctive pattern of brightening, with a peak several weeks after the initial explosion, powered by the decay of 56Ni formed when the core collapsed.

Normally, we can't detect supernovae out to high enough red-shifts to spot them in the early Universe, but the authors of the new paper note that these pair-instability events are extremely bright, saying, "The extreme luminosities of superluminous supernovae extend the redshift limit for supernova detection using present technology." They went back and examined a survey done from the years 2003-2008 called the Canada-France-Hawaii Telescope Legacy Survey Deep Fields. That ran for six months every year from 2003 to 2008. The authors simply looked for objects that weren't present in the first observations but appeared in later ones, and were bright in the ultraviolet at the source (though redshifted to longer wavelengths by the time they reach Earth).

As expected from pair-instability supernovae, these slowly climb to peak brightness, and that peak brightness is enormous: 1044 ergs/second, or 1037 watts. The authors estimate the progenitor of one of these explosions weighed in at 250 solar masses. The other one reached levels of brightness in the UV that aren't possible from the supernova alone, which suggests the explosion ran into gas that had been ejected from the star earlier.

The redshift values of these objects are 2.1 and 3.9, which places them at a time when the Universe was 3 billion and 1.5 billion years old. Follow-up observations with the Keck telescope suggest there were heavier elements around, which indicate these stars were not the first to form in the Universe.

It's hard to extrapolate too much with only two examples, but if the rate of discovery holds up, the authors say that pair instability supernovae may have occurred at a rate 10 times higher than they do in the current Universe. They say that a continued search for these events, along with follow-up observations that check their composition in greater detail, may ultimately reveal the details of the explosion of one of the Universe's first stars.

Promoted Comments

Antimatter created in the first stars because they had to be a few hundred solar masses in order to work? The Universe really is an awesome place. Did I read this right that there'd be no black hole left behind?

A black hole occurs because the rate of fusion at the star's core isn't able to supply the energy needed to hold up the upper layers of the star (due to the fact that it's utterly exhausted it's supply of fuel), which collapse onto the core and force it beyond the density limit required to create a black hole. When the upper layers collapse onto the core, they rebound, even as the core continues to collapse into the black hole, and in addition they're driven off by a massive surge of neutrinos released by the core as it collapses (this neutrino release is actually how the core releases enough energy to keep collapsing). In a pair-instability supernova, the core retains plenty of fuel, but the pair production process short-circuits the mechanism that supports the upper layers of the star, however, the core continues to fuse. As the upper layers collapse, they compress and heat the core even further, which makes the core burn fuel even faster, yet as it gets hotter and hotter more of energy produced is lost to pair-production. Thus the process continues to accelerate until the core gets so dense and so hot that it essentially fuses all of its fuel simultaneously, and the instantaneous release of such massive amounts of energy completely blow the star apart.

Antimatter created in the first stars because they had to be a few hundred solar masses in order to work? The Universe really is an awesome place. Did I read this right that there'd be no black hole left behind?

Very interesting that the very largest and heaviest stars must necessarily explode so thoroughly they leave no black hole behind. In a universe where this wasn't the case, we might never have made it past the first generation or two of stars before everything coalesced into giant black holes.

Antimatter created in the first stars because they had to be a few hundred solar masses in order to work? The Universe really is an awesome place. Did I read this right that there'd be no black hole left behind?

Probably too much energy being thrown about in that thermonuclear explosion for gravity to overcome.

Antimatter created in the first stars because they had to be a few hundred solar masses in order to work? The Universe really is an awesome place. Did I read this right that there'd be no black hole left behind?

A black hole occurs because the rate of fusion at the star's core isn't able to supply the energy needed to hold up the upper layers of the star (due to the fact that it's utterly exhausted it's supply of fuel), which collapse onto the core and force it beyond the density limit required to create a black hole. When the upper layers collapse onto the core, they rebound, even as the core continues to collapse into the black hole, and in addition they're driven off by a massive surge of neutrinos released by the core as it collapses (this neutrino release is actually how the core releases enough energy to keep collapsing). In a pair-instability supernova, the core retains plenty of fuel, but the pair production process short-circuits the mechanism that supports the upper layers of the star, however, the core continues to fuse. As the upper layers collapse, they compress and heat the core even further, which makes the core burn fuel even faster, yet as it gets hotter and hotter more of energy produced is lost to pair-production. Thus the process continues to accelerate until the core gets so dense and so hot that it essentially fuses all of its fuel simultaneously, and the instantaneous release of such massive amounts of energy completely blow the star apart.

Very interesting that the very largest and heaviest stars must necessarily explode so thoroughly they leave no black hole behind. In a universe where this wasn't the case, we might never have made it past the first generation or two of stars before everything coalesced into giant black holes.

"Explode" probably isn't the word for it. There's no shockwave. No core rebound. No sudden collapse.

What happens is essentially as Bad Monkey! has already put it: The star is so massive that its core is so hot that nuclear reactions produce gamma rays at such a high energy that they condense into electron/positron pairs. This reduces the amount of support the energy is giving to the star, so the outer envelope pushes down even harder, increasing both temperature and pressure.

At higher temperature and pressure, the core produces gamma rays with even higher energies, and more of them, so produces more particle pairs, so produces less supporting pressure. The harder that gravity pushes on the core, the less the core pushes back, the more energy it makes.

While we usually talk about millions of years in stellar lifespans, the pair production process takes a week from the first step of the feedback loop to one of the most spectacular events in the universe, where the core is producing energy so ferociously that it exceeds the gravitational binding energy of the entire hundred solar mass star. The star essentially boils itself away with such ferocity that the entire star is completely disrupted.

A magnificent example of the laws of physics, usually so conducive at balancing gravity with fusion to hold a star up, allowing fusion one of its very rare wins.

Hat Monster: What's basically happening here is that the gravitational energy from the collapse is being transformed into matter and radiation, correct?

No. The energy is coming from the core hydrogen (and, at those kinds of conditions, anything else as far as Fe-56), as in any other star. There isn't really that much collapse, just an increase in pressure and temperature.

Hat Monster: What's basically happening here is that the gravitational energy from the collapse is being transformed into matter and radiation, correct?

No. The energy is coming from the core hydrogen (and, at those kinds of conditions, anything else as far as Fe-56), as in any other star. There isn't really that much collapse, just an increase in pressure and temperature.

I see. I appreciate the clarification. Would this sort of "boil off" stop at producing iron, or would there be heavier elements produced? It seems like these early supermassive stars would seed the universe with elements needed for life very quickly.

Can someone point me to an up-to-date book on massive stars and supernovae? I'd prefer it to be relatively light in formulae (more in the "popular" vein, I don't want a text book) and available as an e-book.

Can someone point me to an up-to-date book on massive stars and supernovae? I'd prefer it to be relatively light in formulae (more in the "popular" vein, I don't want a text book) and available as an e-book.

I see. I appreciate the clarification. Would this sort of "boil off" stop at producing iron, or would there be heavier elements produced? It seems like these early supermassive stars would seed the universe with elements needed for life very quickly.

If memory serves, getting to the point of creating iron is usually the death sentence for a star. At that point fusion stops -> the core gets massively compressed by gravity, and the star goes supernova. In the process, one last set of fusion events happen, resulting in gold, uranium and other heavy elements, which is why those are comparatively rare.

I see. I appreciate the clarification. Would this sort of "boil off" stop at producing iron, or would there be heavier elements produced? It seems like these early supermassive stars would seed the universe with elements needed for life very quickly.

If memory serves, getting to the point of creating iron is usually the death sentence for a star. At that point fusion stops -> the core gets massively compressed by gravity, and the star goes supernova. In the process, one last set of fusion events happen, resulting in gold, uranium and other heavy elements, which is why those are comparatively rare.

Yeah, the star fuses at most up to iron and nickel, and then while exploding, pelts its outer layers in the butt with neutrons on the way out. This creates extremely massive unstable isotopes that decay back into the heavy elements like lead and gold that we know and love. I believe the heaviest element ever detected in supernova emissions with certainty is bismuth, but production of anything up to the einsteinium region is plausible.

I see. I appreciate the clarification. Would this sort of "boil off" stop at producing iron, or would there be heavier elements produced? It seems like these early supermassive stars would seed the universe with elements needed for life very quickly.

If memory serves, getting to the point of creating iron is usually the death sentence for a star. At that point fusion stops -> the core gets massively compressed by gravity, and the star goes supernova. In the process, one last set of fusion events happen, resulting in gold, uranium and other heavy elements, which is why those are comparatively rare.

I'm just pondering the difference between this type of star, which basically boils away instead of exploding, versus the Type IA supernovae that result in neutron stars or black holes. If the core keeps fusing like explained, it doesn't collapse. When that "final flash" occurs as the entirety of the core fuses and boils away, what are the elements produced? Do we get more than iron?

I'm just pondering the difference between this type of star, which basically boils away instead of exploding, versus the Type IA supernovae that result in neutron stars or black holes. If the core keeps fusing like explained, it doesn't collapse. When that "final flash" occurs as the entirety of the core fuses and boils away, what are the elements produced? Do we get more than iron?

Type I supernovae are white dwarves blowing themselves apart with runaway fusion after accreting matter past the Chandrasekhar limit. It's type IIs (8+ solar mass star core collapse) that produce black holes and neutron stars, and die a few moments after they fuse their first iron. You don't get past iron in a pair instability explosion. In fact, you typically don't even get there. The star blows itself apart well before the end of its fusion capabilities. On the way out, neutrons convert the star's materials into very high mass atoms that do pass iron, but nothing past iron is directly fused.

There is something puzzling here. The anti-particle pair is generated from gamma rays. But, the antiparticles almost immediately will meet their opposite and be destroyed ..

back into gamma rays.

Huh? How does that reduce the radiation pressure? And the particles most likely to do this are electron positron pairs (the first ones on the stair of size) which I don't believe would generate neutrinos as a side effect of pair annihilation. Do they?

To get energy out of the core without pressure there needs to be a step which generates neutrinos.

Hmm, I guess neutrinos are now understood to have mass, and would be even smaller than electrons, so is what happens here that the neutrino pairs are the ones created directly, and they just high-tail it without colliding? Now that would make more sense.

How about the neutrino pair production? Since they have mass, they could be produced directly from gamma rays, right? Is there some reason that process is not favored? Now, I'm curious.

They cannot be produced from gamma rays. Neutrinos are not charged under the photon (that's what it means to be electrically neutral). Because neutrinos don't feel the photon, they're hard to detect. The only particle that can pair produce neutrinos is the Z boson and, well, it only exists inside particle colliders.

Thus the process continues to accelerate until the core gets so dense and so hot that it essentially fuses all of its fuel simultaneously, and the instantaneous release of such massive amounts of energy completely blow the star apart.

How fast is "simultaneous" and "instantaneous"? Are talking milliseconds, seconds or days?

On a slightly related note, what does the article mean by: They also have a distinctive pattern of brightening, with a peak several weeks after the initial explosion, powered by the decay of 56Ni formed when the core collapsed.

Does that mean the maximum brightness actually occurs due to this process, or that after the initial peak, there is a later, distinct peak caused by this decay?

On a slightly related note, what does the article mean by: They also have a distinctive pattern of brightening, with a peak several weeks after the initial explosion, powered by the decay of 56Ni formed when the core collapsed.

Does that mean the maximum brightness actually occurs due to this process, or that after the initial peak, there is a later, distinct peak caused by this decay?

Much of a supernova's energy is invested throwing neutrons onto nuclei that *do not want them* (most actually goes to neutrinos). These nuclei decay later and power an afterburning more luminous than the initial explosion (I think partly because the collection of gas and dust is simply larger, so it can be brighter later on). The initial explosion is extremely energetic, of course, but a lot of that goes to neutrinos and simply overcoming the entire star's gravitational binding energy (needless to say, enormous...it'd take a really, really big Death Star to blow this thing up). Later on, the energy is free to be emitted as light.

Antimatter created in the first stars because they had to be a few hundred solar masses in order to work? The Universe really is an awesome place. Did I read this right that there'd be no black hole left behind?

A black hole occurs because the rate of fusion at the star's core isn't able to supply the energy needed to hold up the upper layers of the star (due to the fact that it's utterly exhausted it's supply of fuel), which collapse onto the core and force it beyond the density limit required to create a black hole. When the upper layers collapse onto the core, they rebound, even as the core continues to collapse into the black hole, and in addition they're driven off by a massive surge of neutrinos released by the core as it collapses (this neutrino release is actually how the core releases enough energy to keep collapsing). In a pair-instability supernova, the core retains plenty of fuel, but the pair production process short-circuits the mechanism that supports the upper layers of the star, however, the core continues to fuse. As the upper layers collapse, they compress and heat the core even further, which makes the core burn fuel even faster, yet as it gets hotter and hotter more of energy produced is lost to pair-production. Thus the process continues to accelerate until the core gets so dense and so hot that it essentially fuses all of its fuel simultaneously, and the instantaneous release of such massive amounts of energy completely blow the star apart.

Thanks. It kind of takes the phrase "it is better to burn out than fade away" to a whole new level.

Thus the process continues to accelerate until the core gets so dense and so hot that it essentially fuses all of its fuel simultaneously, and the instantaneous release of such massive amounts of energy completely blow the star apart.

How fast is "simultaneous" and "instantaneous"? Are talking milliseconds, seconds or days?

Like Hat Monster said, from the first instability to when the star blows itself apart is days. I'm sure the transition between the star collapsing and then blowing itself apart is probably only minutes, but when you're talking about something that's 150 times the mass of the Sun or more with a diameter the better part of the Earth's orbit, for all intents and purposes that's instantaneous and simultaneous.

Quote:

On a slightly related note, what does the article mean by: They also have a distinctive pattern of brightening, with a peak several weeks after the initial explosion, powered by the decay of 56Ni formed when the core collapsed.

Does that mean the maximum brightness actually occurs due to this process, or that after the initial peak, there is a later, distinct peak caused by this decay?

Most of the energy of the supernova in invested into the kinetic energy of the mass of the star, not light. However, within that cloud of rapidly expanding stellar material, you've got 40 - 50 solar masses of highly radioactive 56Ni, which decays over a period of days into 56Co, which decays over a period of months into stable iron, and emits enough high energy xray radiation in the meantime to superheat the supernova remnants into incandescence